pkg/expect: avoid hardcoding when checking ErrProcessDone

ExpectProcess's Stop method uses 'strings.Contains' to check
the returned err, however, this can be avoided. os.ErrProcessDone's
error message is the same as the hardcoded string. So I think
this explicit error is what this method wants to compare.

Signed-off-by: Jes Cok <xigua67damn@gmail.com>

pkg/expect/expect.go

@@ -286,7 +286,7 @@ func (ep *ExpectProcess) ExitError() error {
 // Stop signals the process to terminate via SIGTERM
 func (ep *ExpectProcess) Stop() error {
 	err := ep.Signal(syscall.SIGTERM)
-	if err != nil && strings.Contains(err.Error(), "os: process already finished") {
+	if err != nil && errors.Is(err, os.ErrProcessDone) {
 		return nil
 	}
 	return err